Aerodynamic blade with high stiffness-to-weight ratio

ABSTRACT

A flight blade or wing having a structural spar at its forward end comprising a solid forward-portion and an after-portion of low density construction, such as a sandwich construction of honeycomb, plastic foam, or balsa wood core and having a structural trailing edge pocket member to produce a blade of maximum stiffness and minimum weight by eliminating leading edge counterweights and reducing spar weight by substituting sandwich construction material for solid metal wall members in the spar after portion.

[ Jan. 30, 1973 154] AERODYNAMIC BLADE WITH HIGH STIFFNESS-TO-WEIGHTRATIO [75] Inventors: Evan A. Fradenburgh, Fairfield; Ed-

mond F. Kiely, Stratford, both of Conn.

United Aircraft Corporation, East [73] Assignee:

Hartford, Conn.

[22] Filed: Oct. 29, 1970 [21] Appl. No.: 85,186

[52] U.S.Cl ..4l6/87,415/226 [51] Int. Cl ..B64c 11/28 [58] Field ofSearch ..4l6/87, 144, 145, 226

[56] References Cited UNITE-D STATESPATE NTS 2,674,327 4/1954 Pullin etal. ..4l6/226 2,580,363 12/1951 Schnitt ..4l6/226 3,155,166 11/1964Stulen et al ..4l6/226 3,249,160 5/1966 M'esserschmitt ..4l6/87 2,734,586 2/1956 Wright et a1 ..4l6/226 3,123,144 3/1964 Stulen et al ..4l6/2263,002,567 lO/l96l Stulen et al..... 416/226 3,528,753 9/1970 Dutton etal. 416/226 2,552,727 5/1951 Lightfoot i. ....4l6/l44 2,574,651 11/1951Meyers ..'...4l6/226 FOREIGN PATENTS OR APPLICATIONS 1,059,072 5/1954France ..4l6/87 Primary Examiner-Everette A. Powell, Jr. A!t0rneyVernonF. Hauschild [57] ABSTRACT A flight blade or wing having a structuralspar at its forward end comprising a solid forward-portion and 10Claims, 7 Drawing Figures PATENTEDJAN 30 I975 SHEET 1 OF 3 H v 6 z WL 5D NE 0 w M 1W flaw m I AERODYNAMIC BLADE WITH HIGH STIFFNESS- TO-WEIGIITRATIO The invention herein described was made in the course of or undercontract or subcontract thereunder with the Department of the Army.

BACKGROUND OF THE INVENTION This invention relates to aerodynamic bladesor wings and more particularly to helicopter blades which have astructural spar defining the blade leading edge and have at least onepocket member extending rearwardly thereof to cooperate therewith indefining the blade airfoil cross-section.

DESCRIPTION OF THE PRIOR ART In the prior art, for example as shown inU.S. Pat. Nos. 2,754,9l7;'2,754,9l8; and 3,323,597, it is conventionalto fabricate this type of helicopter blade so that the blade center ofgravity, the flexural axis in edgewise bending, the aerodynamic center,and the blade feathering axis are located coincident with the bladequarter chord location. The structural spar is fabricated to be hollowand of substantially D-shaped cross section and terminates somewhere aftof the blade quarter chord location and forward of the blade trailingedge so as to define the blade leading edge and forward-portion.Trailing edge pocket members are attached to the spar and cooperatetherewith in defining the blade airfoil cross section.

In the prior art, these trailing edge pockets are made to benonstructural (nonload carrying) to eliminate blade trailing edgecracking problems and they must be counterbalanced by the addition ofnonstructural counterweights in the blade leading edge, preferably inthe spar interior, if these aforementioned axes and stations are toremain coincident. These nonstructural trailing edge pockets andcounterweights add to the centrifugal force which must be carried by theblade during blade rotation but do not assist in preventing flatwisebending. As used herein, flatwise bending means blade bending about theblade axis extending between the blade leading edge and trailing edge.

Certain constructions of this type of blade have been found to havevibratory stress problems in forward flight during certain flightregimes.

SUMMARY OF THE INVENTION A primary object of the present invention is toincrease the stiffness-to-weight ratio of a helicopter blade of thestructural spar-trailing edge pocket type.

In accordance with the present invention, blade weight reduction isaccomplished by reducing the weight of the spar aft of the blade quarterchord station, and thereby providing the added advantage of permittingthe elimination of the leading edge counterweights of the prior artblades.

A further advantage of our invention is increased cross-sectionalinternal area within the structural spar to thereby provide greaterspace for blade control equipment therein.

In accordance with a further aspect of the present invention, the bladestiffness is increased by substituting a structural pocket member forthe prior art nonstructural pockets.

In accordance with a further aspect of the present invention, thestructural spar after-portion is reduced in weight by fabricating thespar after-portion of a sandwich construction having inner and outerskin members and having a core material of low. density, but withadequate dimensional stability and shear strength capability, such ashoneycomb, plastic foam or balsa wood, in place of the prior artconstruction of a solid spar after-portion wall.

In accordance with still another feature of the present invention, thestructural spar can remain solid walled at the areas of high load orstress concentration, for example, at the root end of the spar where thespar connects to the rotor hub and also possibly at the tip end of thespar where the centrifugal loads of the spar are carried in variablediameter rotor installations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of aconventional helicopter blade utilizing our invention.

FIG. 2 is a plan view in cross section illustrating a variable lengthblade.

FIG. 3 is a perspective showing of a prior art blade.

FIG. 4 is a cross-sectional showing of a prior art blade.

FIG. 5 is a sectional view taken along line 5-5 of FIG. 2.

FIG. 6 is an enlarged view of a portion of FIG. 5.

FIG. -7 is a perspective showing of a helicopter blade to betterillustrate our invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 we seehelicopter rotor 10 which consists of a plurality of rotor blades orrotary wings 12 supported by and extending from rotor hub 14 forrotation therewith about axis of rotation 16. Blade 12 consists ofstructural spar 18 and pocket member 20, which is connected thereto soas to form the airfoil cross section of the blade. Spar 18 is connectedin conventional fashion to rotor 14 by connecting mechanism 22. Tip cap24 is positioned at the outer end of spar 18 and pocket member 20 as asmooth aerodynamic closure therefor.

Blade 12 consists of root portion 26, which attaches to hub 14, and tipportion 28, which carries cap 24. The blade spar extends between theblade root 26 and the blade tip 28. Blade 12 further includes leadingedge or forward end 30 and trailing edge or after end 32, and the bladechord extends therebetween. Rotor 10 and blade 12 may be of the typemore particularly described in U.S. Pat. Nos. 3,097,701; 2,754,918; and3,323,597. I

In addition to being used on conventional blade 12 of FIG. 1, ourinvention can also be used with the variable length blade 34 shown inFIG. 2. A plurality of variable length blades, such as 34 of FIG. 2, arepositioned for rotation about a helicopter hub and as the blades arecaused to vary in length as described hereinafter, the diameter of therotor varies. Blade 34 consists of structural spar 18 to whichstructural pocket member 20'attaches and cooperates to define the bladecross-sectional shape shown in FIG. 5.

Torque tube 36 is supported from a rotor hub, such as 14, for rotationtherewith. Torque tube or fixed blade portion 36 supports blade spar 18for translation therealong to cause the blade to change length. Supportbearings 40 and 42 extend between torque tube 36 and blade spar 18 toallow the blade spar and hence the movable portion 44 of the blade,which consists of spar l8, pocket member 20, and tip cap 24, totranslate along axis 46 with respect thereto, when blade length isvaried. The actuation of blade 34 so as to vary the length thereof iscaused in conventional fashion by threaded jackshaft 48 which is mountedand driven for rotation about axis 46 and which engages threaded nut 50so that rotation of jackshaft 48 causes nut 50 to move either leftwardlyor rightwardly on the jackshaft as viewed in FIG. 2, depending upon thedirection of rotation of the jackshaft. Strap member 52 and 54 extendbetween nut50 and plate 56, so that as nut 50 moves tothe right onjackshaft 48 in the FIG. 2 environment, centrifugal force will causeblade moveable portion 44 to move to the right with respect to torquetube 36 and thereby increase the length of blade 34. When nut 50 iscaused to move to the left on jackshaft 48, this movement of nut 50causes strap members 52 and 54 to move the blade movable portion 44leftwardly in the FIG. 2 environment, thereby reducing the length ofblade 34.-Variable length blade 34 may be of the type taught in andactuated as taught in U.S. Pat. Nos. 2,163,482; 3,128,829; and3,249,160.

With the FIG. 1 and 2 blade construction in mind, it is deemed best toconsider the prior art blade construction prior to going into a morecomplete description of our invention, so as to aid the description ofour invention.

Viewing FIGS. 3 and 4, we see the prior art blade 12, which consists ofstructural spar 18' and a plurality of nonstructural pockets connectedthereto and extending rearwardly or aft therefrom to cooperate therewithin defining the airfoil cross section of blade 12'. Spar 18' is ofD-shape cross-section and the walls thereof are made of. solid,homogeneous metal, such as aluminum. The trailing edge pockets 20'consist of a plurality of nonstructural pockets adhesively bonded tospar l8 and extending rearwardly thereof to define blade trailing edge32. This prior art blade 12 is fabricated so that the blade center ofgravity, the blade flexural axis in edgewise bending, the bladeaerodynamic center, and the blade feathering axis are coincident at theblade quarter chord location 60. Spar 18' is hollow and has solid wallsmade of some metal, such as aluminum, and is preferably made as anextrusion and terminates at rear wall 62, the location of which ischosen to fix blade flexural axis at the desired point. The trailingedge pockets 20', of nonstructural construction, are attached theretoand extend rearwardly or aftthereof and serve to cooperate therewith incompleting the airfoil cross-sectional shape of the blade 12. Theaddition of the nonstructural pockets 20' to spar 18' causes the bladecenter of gravity to shift rearwardly or aft of quarter chord station 60and, to bring the blade center of gravity back into coincidence withstation 60, nonstructural leading edge counterweights 64 are bonded intoor in other appropriate ways secured to the interior of theforwardportion of sparl8. These nonstructural members, pockets 20'v andcounterweights 64, add to the centrifugal force acting upon the bladeduring blade rotation but do nothing to stiffen the blade or strengthenthe blade against either flatwise or edgewise bending, that is, bendingout of the plane of rotation or in the plane of rotation.

It has been found that in certain blade constructions of this prior arttype and under particular flight regimes, this blade bending producesvibratory stresses during forward flight and it is therefore the objectof our invention to increase the blade stiffness-to-weight ratio so asto reduce these vibratory stresses. More precisely, our objective is toeither increase the stiffness of the blade or reduce its weight, orboth, so that the centrifugal force causing the aforementioned bendingis reduced as a result of the weight reduction.

Our first step toward weight reduction in our invention is to remove thenonstructural counterweights 64 from the blade spar leading edge. Withthese nonstructural counterweights removed, the blade center of gravityhas moved aft or rearwardly of station 60 and we must therefore balancethe blade by removing weight aft or rearwardly of the quarter chord line60. It is not feasible to attempt to reduce the weight of the trailingedge pockets 20 because they are already at minimum gauge to withstandaerodynamic and blade fluttering loading. We must therefore reduce theweight of the spar after-portion 66 which is aft of the quarter chordline 60. Since it is our objective to bring the ,blade back into balanceand return the blade center of gravity to the quarter chord line 60, ourobjective will be to remove as much weight from the spar afterportion 66asis necessary to counterbalance the loss of the nonstructuralcounterweights 64. We could conceivably do this by reducing the wallthickness of the walls of the spar after-portion 66, however, suchreduced thickness walls would be unable to withstand the endwise andflatwise moments imparted to the spar walls by blade loading. It isaccordingly our teaching to use a sandwich construction to form theblade afterportion 66 since such a construction is light in weight andis capable of withstanding these deforming blade forces. The particularsof this sandwich construction of the blade after-portion 66 will bedescribed in greater particularity hereinafter.

By removing the blade counterweights 64 and substituting a sandwichconstruction for the solid metal wall construction of the sparafter-portion 66, we have reduced the overall weight of the blade 12 andhave probably slightly increased its stiffness-to-weight ratio, however,because the forward-portion 68 of the spar, which is still made of solidmetal, is now stiffer than the after sandwich construction portion 66 ofthe spar, the spar flexural axis has shifted forward of the quarterchord line 60. To bring the flexural axis into coincidence with thequarter chord line 60, the continuous structural trailing edge member 20will be substituted for the plurality of nonstructural pockets 20 of theprior art FIG. 3 construction. By making these changes to the-prior artshown in FIG. 3, a helicopter blade of the spar-pocket type is producedwhich has a substantially greater stiffness-to-weight ratio and istherefore able to withstand the vibratory stresses created by theflatwise and edgewise bending moments encountered during forward flight.

Our blade construction is best described by reference to thecross-sectional showing thereof shown in FIGS. 5 and 6. The samereference numerals previously used will be used to describe the FIGS. 5and 6 construction. Blade 12, our preferred embodiment, includes bladestructural spar l8 and structural trailing edge member which cooperatesto define the blade airfoil cross section concentrically about chordline 70. l-Iollow spar 18 envelopes torque tube 36, which in turnenvelopes actuating jackshaft 48 and nut 50 shown in FIG. 2. Torque tube36, jackshaft 48, and nut 50 are preferably concentric about quarterchord line 60 with which the blade center of gravity, flexural axis inedgewise bending, aerodynamic center and feathering axis are coincident.Spar 18 consists of forward-portion 68, is preferably C-shaped in crosssection and has walls of solid metal, such as aluminum, and the curvedportion thereof defines the blade leading edge 30. The bladeafter-portion 66 is of sandwich construction and is channel-shaped andis structurally bonded to the blade forward-portion 68 and cooperatestherewith to define structural spar 18 so as to be D-shaped in crosssection. The sandwich construction of after spar portion 66 comprisesouter skin 76, inner skin 78, and central core material 80 which extendstherebetween and connects thereto in any preferred fashion, such asbonding. The core material 80 must be low density, dimensionally stableand must have adequate shear strength capabilities. We find that metalhoneycomb, plastic foam, and balsa wood are suitable core materials.Preferably, outer skin 76 and inner skin 78 will be made of the samematerial as the spar forward-portion 68, possibly aluminum, as will thecore 80 of the sandwich construction when a honeycomb core is used. Itwill be evident to those skilled in the art that skin materials ofdifferent moduli than the spar forwardportion 68 could be used to effectfine adjustments in the location of the flexural axis. Trailing edgepocket 20 includes a tapered honeycomb core 82 sandwiched between skinmembers 84 and 86, which may be made of aluminum, and which areadhesively bonded at their forward ends to spar l8 and which areadhesively bonded to and envelope blade trailing edge strip 88 at theirafter ends where they cooperate therewith in defining the blade trailingedge 32.

Referring to FIG. 7 we see a perspective showing of blade 12incorporating our invention and partially broken away to illustratecertain features of our invention. As previously described, blade 12comprises nonhomogeneous spar 18 to which structural pocket member 20 isattached to form the blade airfoil cross section. Spar 18 consists ofspar forward-portion 68, which is of solid wall construction andafter-portion 66, which is of sandwich-type wall construction. As bestillustrated in FIG. 7, under certain circumstances it may be desirableto maintain solid wall construction throughout the full cross section ofspar 18 in regions of high load or stress. For example, FIG. 7illustrates the spar root portion 90 and the spar tip portion 92 to beof solid wall construction throughout the full spar cross section. It isdeemed desirable to have the root portion 90 of solid constructionbecause of the heavy load which that portion of the spar must carry intransferring loads between the blade and the rotor hub. This isaccordingly probably the preferred construction for all helicopterblades whether of the constant length variety shown in FIG. 1, whereinthe boundary of the spar after-portion is along line 91, or the variablelength variety shown in FIG. 2. In the FIG. 2 variety, it may bedesirable to have the blade tip portion 92 also of the solid wallconstruction because of the concentrated loads which are passedtherethrough between extension members 52 and 54 and the actuating nut50.

From the above description it will be obvious that we have produced ahelicopter blade of high stiffness-toweight ratio by fabricating thehollow structural spar to be of nonhomogeneous, continuous, single wallconstruction with the after-portion of sandwich construction so as to beless dense than the solid forward-portion and thereby permitting theelimination of leading edge counterweights, and preferably alsoutilizing a structural trailing edge member with the spar in preferencewith the conventional nonstructural trailing edge pockets.

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and described, for obviousmodifications will occur to a person skilled in the art.

We claim:

1. A helicopter blade having a blade leading edge and a blade trailingedge and a blade chord extending therebetween and further including ablade root section and blade tip section located at opposite ends of theblade span and including:

A. a hollow structural spar of D-shaped cross section extending alongthe blade span and having'.

1. a forward-portion fabricated of solid metal and being of C-shapedcross section and positioned so that the curved portion of the C definesthe blade leading edge and so that the legs of the C extend aft;

2. an after-portion comprising a wall of sandwich construction and beingof channel-shaped cross section and of lower density than the sparforward-portion and connected thereto to cooperate therewith in defininga hollow spar of D-shaped cross section and which wall comprises:

a. an outer skin,

b. an inner skin,

0. and a low density core extending between and connected to said skins;

B. a structural pocket member positioned aft of said spar and connectedthereto and shaped to define the blade trailing edge and to cooperatewith said spar to define the blade cross-sectional shape.

2. Apparatus according to claim 1 wherein said core is of honeycombconstruction.

3. Apparatus according to claim 1 wherein said core is plastic foam.

4. Apparatus according to claim 1 wherein said core is fabricated frombalsa wood.

5. A helicopter blade having a blade leading edge and a blade trailingedge and a blade chord extending therebetween and further including ablade root section and blade tip section located at opposite ends of theblade span and including:

A. a hollow structural spar of D-shaped cross section extending alongthe blade span and having:

1. a forward-portion fabricated of solid metal and being of C-shapedcross section and positioned so that the curved portion of the C definesthe blade leading edge and so that the legs of the C extend aft,

. an after-portion comprising a wall of sandwich construction and beingof channel-shaped cross section and of lower density than the sparforward-portion and connected thereto to cooperate therewith in defininga hollow spar of D-shaped cross section and which wall comprises:

a. an outer skin,

b. an inner skin,

c. and a low density core extending between and connected to said skins;

B. a structural pocket member positioned aft of said spar and connectedthereto and shaped to define the blade trailing edge and to cooperatewith said spar to define the blade cross-sectional shape and being ofsandwich construction including:

1. an outer skin,

2. an inner skin,

3. a tapered honeycomb core extending between and connected to saidskins.

6. Apparatus according to claim Sand wherein said pocket member includesa trailing edge strip member extending spanwise along said blade andpositioned between said pocket inner and outer skins and cooperatingtherewith to define said blade trailing edge.

7. A helicopter blade having a blade leading edge and a blade trailingedge and a blade chord extending therebetween and further including ablade root section and blade tip section located at opposite ends of theblade spar and including:

A. a hollow structural spar of D-shaped cross section extending alongthe blade span and having:

1. A forward-portion fabricated of solid metal and being of C-shapedcross section and positioned so that the curved portion of the C definesthe blade leading edge and so that the legs of C extend aft,

. an after-portion of channnel-shaped cross section fabricated to be oflower density than the spar forward portion and connected thereto tocooperate therewith in defining a hollow spar of D-shaped cross section,

B. a pocket member positioned aft of said spar and connected thereto andshaped to define the blade trailing edge and to cooperate with said sparto define the blade cross-sectional shape, and

wherein said blade spar includes:

A. a root portion of D-shaped cross section with solid metal walls anddefining the spar forward-portion and the spar after-portion at the sparroot, and

B. a tip portion of D-shaped cross section with solid metal walls anddefining the spar forward-portion and the spar after portion at the spartip.

8. A helicopter blade having a blade leading edge and a blade trailingedge and a blade chord extending therebetween and further including ablade root section and blade tip section located at opposite ends of theblade spar and including:

A. a hollow structural spar of Dshaped cross section extending along theblade span and having:

l. a forward-portion fabricated of solid metal and being of C-shapedcross section and positioned so that the curved portion of the C definesthe blade leading edge and so that the legs of C extend aft,

. an after-portion of channel-shaped cross section fabricated to be oflower density than the spar forward-portion and connected thereto tocooperate therewith in defining a hollow spar of D-shaped cross section,

B. a pocket member positioned aft of said spar and connected thereto andshaped to define the blade trailing edge and to cooperate with said sparto define the blade cross-sectional shape, and

wherein said blade spar includes:

A. a root portion of D-shaped cross section with solid metal walls anddefining the spar forward-portion and the spar after-portion at the sparroot.

9. A rotary blade hollow structural spar shaped to define the forwardportion of the airfoil shape of the blade and including:

A. a solid walled forward-portion extending along the blade span anddefining the blade leading edge,

B. a sandwich construction, walled after-portion attached to and shapedto cooperate with said forward-portion to form the blade structural sparand including:

1. an outer skin,

2. an inner skin,

3. lightweight core material extending between and connected to saidinner and outer skin, and

C. wherein said spar forward-portion is of selected wall thickness andof C-shaped cross section and positioned so that the curved portion ofthe C defines the blade leading edge and so that the legs of C extendaft therefrom and, further, wherein the spar after-portion is ofsubstantially the same wall thickness as said wall thickness of saidforwardportion and is of channel-shaped cross section with the legs ofthe channel connected directly to the legs of the spar forward-portionto form a smooth continuation thereof so that the spar forward andafter-portions cooperate to define a hollow spar of D-shaped crosssection and substantially constant wall thickness.

10. A variable length aerodynamic blade including:

A. a fixed blade portion adapted to connect to a driving hub forrotation therewith,

B. a movable blade portion mounted on said fixed blade portion formovement with respect thereto so as to vary the blade length andincluding:

1. a hollow structural spar of D-shaped cross section extending alongthe blade span and having: a. a forward-portion fabricated of solidmetal and being of C-shaped cross section and positioned so that thecurved portion of the C defines the blade leading edge and so that thelegs of the C extend aft,

. an after-portion comprising a wall of sandwich construction and beingof channelshaped cross section and of lower density than the sparforward-portion and connected thereto to cooperate therewith in defininga hollow spar of D-shaped cross section and which wall com prises:

1. an outer skin,

2. an inner skin,

3. and a low density core extending between shape, and

and connected skms, C. means to cause said movable blade portion to 2. astructural pocket member positioned aft of said spar and connectedthereto and shaped to define the blade trailing edge and to cooperatewith said spar to define the blade cross-sectional move relative to saidfixed blade portion so as to vary blade length.

1. A helicopter blade having a blade leading edge and a blade trailingedge and a blade chord extending therebetween and further including ablade root section and blade tip section located at opposite ends of theblade span and including: A. a hollow structural spar of D-shaped crosssection extending along the blade span and having:
 1. a forward-portionfabricated of solid metal and being of Cshaped cross section andpositioned so that the curved portion of the C defines the blade leadingedge and so that the legs of the C extend aft;
 2. an after-portioncomprising a wall of sandwich construction and being of channel-shapedcross section and of lower density than the spar forward-portion andconnected thereto to cooperate therewith in defining a hollow spar ofD-shaped cross section and which wall comprises: a. an outer skin, b. aninner skin, c. and a low density core extending between and connected tosaid skins; B. a structural pocket member positioned aft of said sparand connected thereto and shaped to define the blade trailing edge andto cooperate With said spar to define the blade crosssectional shape. 1.A helicopter blade having a blade leading edge and a blade trailing edgeand a blade chord extending therebetween and further including a bladeroot section and blade tip section located at opposite ends of the bladespan and including: A. a hollow structural spar of D-shaped crosssection extending along the blade span and having:
 1. a forward-portionfabricated of solid metal and being of C-shaped cross section andpositioned so that the curved portion of the C defines the blade leadingedge and so that the legs of the C extend aft;
 1. a hollow structuralspar of D-shaped cross section extending along the blade span andhaving: a. a forward-portion fabricated of solid metal and being ofC-shaped cross section and positioned so that the curved portion of theC defines the blade leading edge and so that the legs of the C extendaft, b. an after-portion comprising a wall of sandwich construction andbeing of channel-shaped cross section and of lower density than the sparforward-portion and connected thereto to cooperate therewith in defininga hollow spar of D-shaped cross section and which wall comprises:
 1. anouter skin,
 1. a forward-portion fabricated of solid metal and being ofC-shaped cross section and positioned so that the curved portion of theC defines the blade leading edge and so that the legs of the C extendaft,
 1. a forward-portion fabricated of solid metal and being ofC-shaped cross section and positioned so that the curved portion of theC defines the blade leading edge and so that the legs of C extendaft,
 1. an outer skin,
 1. an outer skin,
 1. A forward-portion fabricatedof solid metal and being of C-shaped cross section and positioned sothat the curved portion of the C defines the blade leading edge and sothat the legs of C extend aft,
 2. an inner skin,
 2. an inner skin,
 2. astructural pocket member positioned aft of said spar and connectedthereto and shaped to define the blade trailing edge and to cooperatewith said spar to define the blade cross-sectional shape, and C. meansto cause said movable blade portion to move relative to said fixed bladeportion so as to vary blade length.
 2. an inner skin,
 2. anafter-portion comprising a wall of sandwich construction and being ofchannel-shaped cross section and of lower density than the sparforward-portion and connected thereto to cooperate therewith in defininga hollow spar of D-shaped cross section and which wall comprises: a. anouter skin, b. an inner skin, c. and a low density core extendingbetween and connected to said skins; B. a structural pocket memberpositioned aft of said spar and connected thereto and shaped to definethe blade trailing edge and to cooperate With said spar to define theblade cross-sectional shape.
 2. Apparatus according to claim 1 whereinsaid core is of honeycomb construction.
 2. an after-portion comprising awall of sandwich construction and being of channel-shaped cross sectionand of lower density than the spar forward-portion and connected theretoto cooperate therewith in defining a hollow spar of D-shaped crosssection and which wall comprises: a. an outer skin, b. an inner skin, c.and a low density core extending between and connected to said skins; B.a structural pocket member positioned aft of said spar and connectedthereto and shaped to define the blade trailing edge and to cooperatewith said spar to define the blade cross-sectional shape and being ofsandwich construction including:
 2. an after-portion of channnel-shapedcross section fabricated to be of lower density than the spar forwardportion and connected thereto to cooperate therewith in defining ahollow spar of D-shaped cross section, B. a pocket member positioned aftof said spar and connected thereto and shaped to define the bladetrailing edge and to cooperate with said spar to define the bladecross-sectional shape, and wherein said blade spar includes: A. a rootportion of D-shaped cross section with solid metal walls and definingthe spar forward-portion and the spar after-portion at the spar root,and B. a tip portion of D-shaped cross section with solid metal wallsand defining the spar forward-portion and the spar after portion at thespar tip.
 2. an after-portion of channel-shaped cross section fabricatedto be of lower density than the spar forward-portion and connectedthereto to cooperate therewith in defining a hollow spar of D-shapedcross section, B. a pocket member positioned aft of said spar andconnected thereto and shaped to define the blade trailing edge and tocooperate with said spar to define the blade cross-sectional shape, andwherein said blade spar includes: A. a root portion of D-shaped crosssection with solid metal walls and defining the spar forward-portion andthe spar after-portion at the spar root.
 3. Apparatus according to claim1 wherein said core is plastic foam.
 3. a tapered honeycomb coreextending between and connected to said skins.
 3. and a low density coreextending between and connected to said skins,
 3. lightweight corematerial extending between and connected to said inner and outer skin,and C. wherein said spar forward-portion is of selected wall thicknessand of C-shaped cross section and positioned so that the curved portionof the C defines the blade leading edge and so that the legs of C extendaft therefrom and, further, wherein the spar after-portion is ofsubstantially the same wall thickness as said wall thickness of saidforward-portion and is of channel-shaped cross section with the legs ofthe channel connected directly to the legs of the spar forward-portionto form a smooth continuation thereof so that the spar forward andafter-portions cooperate to define a hollow spar of D-shaped crosssection and substantially constant wall thickness.
 4. Apparatusaccording to claim 1 wherein said core is fabricated from balsa wood. 5.A helicopter blade having a blade leading edge and a blade trailing edgeand a blade chord extending therebetween and further including a bladeroot section and blade tip section located at opposite ends of the bladespan and including: A. a hollow structural spar of D-shaped crosssection extending along the blade span and having:
 6. Apparatusaccording to claim 5 and wherein said pocket member includes a trailingedge strip member extending spanwise along said blade and positionedbetween said pocket inner and outer skins and cooperating therewith todefine said blade trailing edge.
 7. A helicopter blade having a bladeleading edge and a blade trailing edge and a blade chord extendingtherebetween and further including a blade root section and blade tipsection located at opposite ends of the blade spar and including: A. ahollow structural spar of D-shaped cross section extending along theblade span and having:
 8. A helicopter blade having a blade leading edgeand a blade trailing edge and a blade chord extending therebetween andfurther including a blade root section and blade tip section located atopposite ends of the blade spar and including: A. a hollow structuralspar of D-shaped cross section extending along the blade span andhaving:
 9. A rotary blade hollow structural spar shaped to define theforward portion of the airfoil shape of the blade and including: A. asolid walled forward-portion extending along the blade span and definingthe blade leading edge, B. a sandwich construction, walled after-portionattached to and shaped to cooperate with said forward-portion to formthe blade structural spar and including: